Production of manganese dioxide



Patented Aug. 30, 1932 UNITED sr-A'ras PATEN'FOFFICE OLIVER W. STOREY, OI MADISON, WISCONSIN, ASSIGNOB '10 BURGESS BATTERY K PANY, OI MADISON, WISCONSIN A CORPORATION OF WISCONSIN PRODUCTION OI MANGANESE DIOXIDE Rom-swing.

This invention relates to the production of manganese dioxide by an electrolytic method and more particularly to its production from rhodochrosite, a manganese carbonate ore, by

- a cyclic process.

ther object to provide an improved electrolyte for the electrolysis operation.

Rhodochrosite, a'manganese carbonate ore,

is available in large quantities in Montana but it can not be changed directly b any known process into a suitable form 0 high grade manganese oxide since it is necessary to remove the impurities which are associated with it. These impurities usually are silica, calcite, magnesium carbonate, other silicious compounds and heavy metal compounds. If the manganese dioxide is to be used as a depolarizer in dry cells it is essential to remove these heavy metals since the presence of some of them in the depolarizer even in minute quantities would cause rapid deterioration of such cells.

I have discovered that it is possible to dissolve the Montana rhodochrosite ore in dilute sulphuric acid to form a solution containing manganese sulphate, then purify the resulting sulphate solution, and then electrolyze this purified solution to form manganese dioxide and dilute sulphuric acid. This sulphuric acid can be used to treat further quantitiesnf the ore. The process thereby becomes cyclic with the simultaneous production of a high grade of manganese dioxide. It is therefore only necessary to supply as raw material in addition to the ore small amounts of sulphuric acid, to make up the losses in the process, and purification materials. All of the impurities present in appreciable amounts except the magnesium salts may be removed during the purification operations. The magnesium sulphate formed by the sulphuric acid, accumulates in the electrolyte. It is desirable, however, to have this salt present in the electrolyte and it is therefore not necessary to make any provision for its removal. I

In carrying out my invention the man- Application filed Kay 12, 1981. Serial K0. 538,987.

ganese carbonate ore is ground to a powder, preferably to about 100 mesh or finer, to facilitate reaction with the dilute sulphuric acid that is employed to convert the carbonate to manganese sulphate. It is preferable to use an ore that has a low calcite content since the sulphuric acid consumed in dissolving it is wasted, as the resulting insoluble calcium sulphate is discarded. The reaction is preferably carried out at near the boiling point of water in a counter-current system so that the insoluble portion of the ore which is discarded contains a minimum of undissolved manganese compounds. The resulting manganese sulphate solution is practically neutral.

The acid strength ofthe leaching liquor should besuch that the resulting neutral solution preferably contains from 12% to 20% of manganese sulphate. With greater concentrations the solution becomes more diflicult to handle, its purification becomes more difficult, and there is a decrease in the rate of solution of the manganese carbonate. Obviously the solution preferably should not be more dilute than the minimum strength used in the electrolytic cells. The dilute acid coming from the electrolytic cells after electrolysis contains manganese sulphate since the manganese'is not completely removed. Sufiicient sulphuric acid is added to this deleted electrolyte to make up for mechanical osses and losses due to the formation of sulphates other than manganese, due to the presence of impurities in the ore as previously described. Except for the addition of sulphuric acid to cover these losses, the depleted electrolyte contains the correct amount of sulphuric acid to secure the desired manganese sulphate concentration in the leaching operation.

The manganese sulphate solution is decanted or filtered from the insoluble 1 of the ore. The hot solution is then oxidized, preferably by aeration. A regulated amount of finely divided calcium carbonate is em- I5 of it will greatly decrease the current efliciency. Magnesium carbonate or manganese carbonate may also be used to mamtam the neutrality of the solution. The hydrates or oxides may be used instead of the carbonates, but these substances tend to rec1pitate the man anese as a hydrate, wh1ch is oxidized readily to brown oxides. The manganese sulphate solution is filtered and 1s ready to be used in the electrolytic cells after bemg diluted to the desired concentration. The solution should not contain any or only m1- nute amounts of any compounds which will cause reduction of the manganese dioxide formed during electrolysis. Ferrous compounds and-certain organic materials are examples of these reducing com ounds The electrolysis is preferab y carried out in a hot solution along the lines described by Van Arsdale and Maier (see Patent No.

1,304,222 and Trans. Am. Electrochem. Soc., Vol. 33, p. 109, 1918) but modlfied for cyclical operation. Lead lined tanks may be used, the lead preferably being hardened. The anodes may be platlnum or graphite, the latter being impregnated with bakelite or similar water and acid resisting material to prevent rapid disintegration. I prefer, however, to use lead which has been hardened to prevent sagging which occurs in the hot electrolyte when pure lead is used. The hardened anode also facilitates the removal of the manganese dioxide deposit. Lead hardened with about 6% of antimony has given good results. The percentage of antimony may be varied. Various lead alloyssuch as lead-silver, lead-thallium and others may be used. The cathode may ,be of graphite or suitable metals or alloys. The electrolyte should be kept hot, 90 0. being a favorable temperature. A current density of about 6 to 10 amperes per square foot gives good results. Good results have been obtained with an electrolyte containing about molar manganese sulphate, or the equivalent mix- .ture of manganese sulphate and sulphuric.

acid. Under the above conditions of electrolysis the current efiiciency is not decreased by the replacement of the manganese sulphate by its equivalence in sulphuric acid until the manganese sulphate content drops below about molar. The electrolysis may therefore .be conducted at the highest efficiency under wide limits of sulphuric acid and manganese sulphate ratios. It is desirable theoretically for the electrolyte to contain as high a content of manganese sulphate or equivalent mixture with sulphuric acid as possible but practical considerations interfere with the use of saturated or nearly saturated solutions. I have found that an electrolyte containing about 150 grams MnSO. per liter gives excellent results though concentrations of 100 grams to 200 grams per liter may be used. These concentrations include the sulphuric acid resent in terms of equivalent manganese su phate. Above 200 grams per liter the solution becomes too heavy and various difi'iculties are encountered at certain points in the process.

Instead of electrolyzing the purified neutral manganese sulphate solution under the above conditions until the manganese content is practically exhausted to produce dilute sulphuric acid asa by-product as has heretofore been done, this solution is fed into the electrolytic cell continuously at a predetermined rate proportionate to the current. There is withdrawn at another point, a like amount of exhausted electrolyte which contains for example, 30. grams manganese sulphate per liter and 78 grams sulphuric acid per liter, thereby making the deposition continuous in contrast to the above mentioned intermittent process. This exhausted electrolyte is then returned to the first step in the process to be used in the treatment of manganese carbonate ore. Since the electrolysis may be carried out with equal efiiciency in electrolytes containing smaller amounts of manganese sulphate and larger amounts of free acid as previously explained, I prefer to recirculate a large proportion of the exhausted high-acid electrolyte with the incoming neutral sulphate electrolyte so that the electrolysis is conducted with an electrolyte that is practically of uniformly high acid content throughout the cell. Better electrolyte circulation is also secured by this method. In a specific example the incoming neutral electrolyte (taken as one volume) contains 150 grams manganese sulphate per liter and the exhausted electrolyte contains 50 grams manganese sulphate and 66 grams sulphuric acid per liter. Exhausted electrolyte equal in volume to the neutral electrolyte is returned to the leaching operation to react with the ore. However, an additional three volumes of exhausted electrolyte arewithdrawn at the same time and mixed with the incoming neutral electrolyte so that the incoming electrolyte (a mixture of one volume of neutral and three volumes exhausted) is composed of 7 5 grams manganese sulphate and 49 grams sulphuric acid per liter. These proportions are merely illustrative and may vary over a wide range to suit the specific conditions. 4

The manganese dioxide, which is depositedciency apparently results mostly or entirely from the decrease in impressed voltage on the cells. My measurements show that the impressed voltage is decreased from 15% to 30% when magnesium sulphate is added to a hot, pure, neutral manganese sulphate electrolyte. As low as 0.1% of magnesium sulphate has a marked effect. I refer having at least 2% of magnesium su phate in the electrolyte. Excellent results have been obtained with an electrolyte containin about 8% to 12% of magnesium sulphate. igher percentages may be used. Under the conditions of continuous operation described it was found that with a rhodochrosite containing 3.15% magnesium carbonate, the electrolyte seldom contained more than about 10% of magnesium sulphate, probably because of certain unknown equilibrium conditions, al--- though it is possible to dissolve considerably more than this amount of magnesium sulphate in pure manganese sulphate solutions of equal strength. Such solutions containing in excess of 10% of magnesium sulphate maybe electrolyzed to produce manganese dioxide with excellent result.

The manganese carbonate ore usually contains magnesium carbonate, which dissolves in the sulphuric acid of the leaching solution to form magnesium sulphate. There may be more than suflicient carbonate in the ore to maintain the desired amount of the magnesium sulphate in the electrolyte without the extraneous addition of this salt. Because of the probable equilibrium conditions mentioned above the electrolyte does not seem to accumulate an excess of magnesium sulphate where there is not an excessive amount of magnesium carbonate in the ore. It, therefore, does not become necessary to discardthe electrolyte from time to time because of the accumulation of too much magnesium sulphate in it, with the consequent loss of manganese sulphate and sulphuric acid which would be entailed.

I claim:

1. A solution for use in the production of electrolytic manganese dioxide which comprises a water solution of neutral purified manganese sulphate containing magnesium sulphate in an amount suflicient to depress markedly the impressed voltage required for the hot electrolysis of said solution to produce said dioxide.

2. A solution for use in the production of electrolytic manganese dioxide-which comprises a water solution of purified manganese sulphate, said solution containing at least 100 grams per liter of said sulphate and at least 0.1% magnesium sulphate. I

electrolytic manganese dioxide which comprises a water solution of purified manganese sulphate, said solution containing at least 100 grams per liter of combined manganese sulhate and sulphuric acid in termsof equivaent manganese sulphate, but not less than about 30 grams per liter of said sulphate, and at least 2% magnesium sulphate.

4. A solution for use in the production of electrolytic manganese dioxide which comprises a water solution of purified manganese sulphate, said solution containing from about 100 grams to 200 grams per liter of said sulphate, and at least 2% of magnesium sulphate.

5. An electrolyte for the production of electrolytic manganese dioxide which comprises a water solution of substantially ironfree manganese sulphate, said solution containing from 100 grams to 200 vgrams per liter of combined manganese sulphate and sulphuric acid in terms of equivalent manganese sulphate, but not less than about 30 grams per liter of said sulphate, and at least I 2% of magnesium sulphate.

6. The method for appreciably depressing the voltage impressed on a. hot manganese sulphate solution to produce manganese dioxide therefrom, which comprises introducing about 0.1% or more of magnesium sulphate into said electrolyte.

7. In the electrolytic production ofmanganese dioxide the method which comprises electrolyzing a purified hot manganese sulphate solution containing at least 100 grams per liter of combined manganese sulphate and sulphuric acid in terms of manganese sulphate, but not less than about 30 grams per liter of said sulphate, and at least 2% of magnesium sulphate.

8. In the electrolytic production of manganese dioxide the method which comprises electrolyzing a purified hot manganese sulphate solution containing from 100 grams to 200grams per liter of combined manganese sulphate and sulphuric acid in terms of manganese sulphate but not less than about 30 grams per'liter of said sulphate, and at least 0.1% 0 magnesium sulphate.

9. In the electrolytic production of manganese dioxide the method which comprises electrolyzing with a cathode anda hardened lead anode a substantially iron-free hot manganese sulphate solution containing from 100 grams to 200 grams per liter of combined manganese sulphate and sulphuric acid in terms of manganese sulphate but not less than about 30 grams per liter of said sulphate, and at. least 2% of magnesium sulphate.

10. In the cyclical process of continuous electrolytic production of manganese dioxide the method which comprises adding continuously a purified manganese sulphate solution containing magnesium sulphate to the elec- 3. An electrolyte for the production of trolyte to replenish the manganese sul hate content and withdrawing continuous y an equivalent amount of depleted electrolyte electrolytic production of man anese dioxide the method which comprises a ding continuously a purified and substantially iron-free hot manganese sulphate solution containing from 100 grams to 200 grams per liter of manganese sulphate and at least 01% of magnesium sulphate to the electrolyte to re-. plenish the manganese sulphate content and withdrawing continuously an equivalent amount of depleted electrolyte containing magnesium sulphate.

In testimony whereof I aflix my si nature.

OLIVER W. STO EY. 

